Mass spectrometers are used in sample analysis. Mass spectrometry is the process whereby volatile, liquid and solid, organic and inorganic compounds are identified and quantified based on their molecular weight and characteristic fragmentation patterns. Mass spectrometers are often large and heavy. Effective operation of mass spectrometers requires, in most cases, strict control of temperature, humidity, and vibrations. These characteristics, among others, limit the use of mass spectrometers in the field.
Time-of-flight mass spectrometry is an intrinsically simpler approach to mass analysis than the approach used by mass analyzers based on ion cyclotron resonance or devices such as magnetic, and electrostatic sectors and quadrupoles. Scanning analyzers use a magnetic or electrostatic field to sort ions into their respective spectra for analysis by mass spectroscopy. The sorting is accomplished by varying the magnetic or electrical field strength. Hence, at any given time, only one mass may be detected. Time-of-flight spectrometers have the capability of simultaneously detecting the complete mass spectrum.
In time-of-flight mass spectrometry, a sample to be analyzed is introduced into an ionization region. The ions generated in the ionization region are accelerated by an electric field into a drift region. ##EQU1##
Ideally, each ion enters the drift region with a kinetic energy E that is proportional to the charge q of the ion. The proportionality constant V is the potential difference across the ionization region. See equation 1. Each ion's velocity v in the drift region is proportional to the square root of the ion's charge-to-mass ratio. See equation 2. Thus, the time t it takes each ion to traverse the drift region, of length d, which is inversely proportional to the velocity v of the ion, is proportional to the square root of the ion's mass-to-charge ratio. See equation 3. The time interval between the formation of an ion and its arrival at the detector is recorded and the mass-charge ratios are derived therefrom. For gaseous samples, where the ions are produced in a relatively large volume, two acceleration regions may be used to determine a well defined flight time for each charge-to-mass ratio. Time-of-flight analyzers are simpler, more compact, lighter in weight, and more rugged than traditional analyzers and therefore are more suitable for use in the field.
Efforts have been made to combine conventional ionization techniques such as electron impact with time-of-flight analysis. The article, "An Electron Impact Storage Ion Source For Time-Of-Flight Mass Spectrometers", International Journal of Mass Spectrometry and Ion Processes, R. Grix et at., 1989, p. 323-330 ("Grix") discloses an electron impact ion source that continuously produces ions, stores those ion, and releases the ions in bursts of about 10 nanoseconds ("ns") duration. This ion source is then used for time-of-flight mass spectrometry.
Time-of-flight mass spectrometry of ions requires that the ions be gated into the drift region, i.e., introduced as temporal pulses of ions. The conventional method for introducing temporal pulses of ions into the drift region is the pulsed ionization/extraction method. This method requires elaborate pulsers and pulsing schemes which are a major complication for field portable instruments. Moreover, the uncertainty of the variables involved in pulsed ionization/extraction processes results in measurements having relatively poor mass resolution. Mass resolution is the ratio of the mass of a received particle to the error in determining the mass. Because it is a ratio of a mass to a mass, mass resolution is dimensionless. For instance, if a particle is determined to have a mass of twenty-eight atomic mass units give or take a tenth of a mass unit, the mass resolution is 280. Mass resolution in mass spectrometry is important both for determining the identity of the unknown compound and for elimination of other compounds from the set of potential compounds being analyzed. Consequently, the better the mass resolution a mass spectrometer achieves, the more accurate the spectrometer is, and hence, the more useful it is. The pulsed ionization/extraction method may provide an insufficient mass resolution due to relatively long electron pulse widths, on the order of about 10 ns. The uncertainty in the start time of the ion's travel leads to an uncertainty in the overall time-off-flight from which the mass of the ions is calculated. Existing technology does not include a portable electron-impact ion source having a pulse width significantly less than 10 ns.
Accordingly, a need has arisen for a compact, portable mass spectrometer with improved mass resolution and sensitivity.